The invention relates to cardiac pacemaker leads, and more particularly to improvements in cardiac pacemaker leads with fixed tissue securing means. Still more particularly, the invention relates to improvements in cardiac pacemaker leads of the type wherein the tissue securing means (such as a fixation helix of platinum alloy or the like) is prevented from damaging the tissue surrounding a body cavity and/or the heart tissue during implantation of the lead.
A cardiac pacemaker lead normally comprises an elongated flexible dielectric sheath with a proximal end at the pacemaker and a distal end at the locus of anchoring in heart tissue, and a flexible conductor which extends through the sheath and has a securing means or terminal projecting beyond the distal end of the sheath and being anchorable in heart tissue. In many instances, the terminal is a so-called fixation helix with a sharp tip which is caused to penetrate into heart tissue in response to rotation of the lead subsequent to completion of the implanting step, e.g., through a blood vessel.
German patent application No. 25 39 553 of Peter Osypka (published Mar. 10, 1977) discloses a cardiac pacemaker lead wherein the terminal of the flexible conductor extends a fixed distance beyond the distal end of the dielectric sheath and constitutes a helix with a sharp tip which is caused to penetrate into heart tissue as soon as the implanting step is completed. The tip of the helix penetrates into the adjacent tissue in response to rotation of the entire lead. Problems can arise during implantation because the sharp tip of the helix is exposed and can cause injury to tissue surrounding the blood vessel or vessels which are selected for implantation of the distal end of the lead into a patient's heart. Furthermore, the tip of the helix can cause injury to a heart valve on its way toward the locus of penetration into heart tissue. The danger of injury is especially pronounced in regions where the distal end of the lead must change the direction of its advancement toward the heart or toward the locus of anchoring in the heart.
Proposals to avoid injury to tissue during implantation of a cardiac pacemaker lead include the provision of a retractable and extendable conductor, i.e., the helix at the distal end of the conductor is retractable into the adjacent end of the insulating sheath and is expelled from the sheath when the implanting step is completed. This normally involves the provision of mating threads in the distal end of the sheath and on the distal end of the conductor, i.e., the conductor can be rotated relative to the sheath in order to retract the tip of its helix during implantation and to expel the tip of the helix when the implanting step is completed and the tip of the helix is ready to penetrate into heart tissue. Reference may be had, for example, to U.S. Pat. No. 4,106,512 granted Aug. 15, 1978 to Hans-Jurgen Bisping and to U.S. Pat. No. 4,217,913 granted Aug. 19, 1980 to Robert G. Dutcher. The patented proposals exhibit the important advantage that the likelihood of injury to tissue during implantation of the cardiac pacemaker lead is reduced or eliminated, even if the helix at the distal end of the conductor is provided with a sharp tip. However, such proposals also exhibit a serious drawback, namely that the pacemaker lead is much more complex because it must be provided with screw threads in the sheath and with complementary screw threads on the conductor. Such treads are needed only and alone during implantation of the lead and subsequent anchoring of the terminal in heart tissue. Since the transverse dimensions of the distal end of the lead are very small, the mating threads must be machined and/or otherwise formed with a high degree of precision which contributes significantly to the cost of the pacemaker lead. Furthermore, the provision of threads at the distal end contributes to the radial dimensions of the lead and hence to the difficulty of implanting such bulky lead in the body of a patient. The bulky distal end of the implanted lead is likely to constitute a cause of irritation of the adjacent wall in a patient's heart which can result in an increase of sensitivity of heart tissue. Still further, the clearances between the mating external and internal threads are likely to establish paths for undesirable escape of blood. In other words, the advantage of avoiding potential injury to tissue during implantation of the cardiac pacemaker lead is achieved by accepting a number of disadvantages including higher cost, more pronounced irritation during implantation due to greater bulk of the distal end of the lead, increased sensitivity of the tissue at the locus of anchoring of the distal end of the electrode, and eventual leakage of body fluid.
European patent application No. A 0 219 608 of (published ) discloses a cardiac pacemaker lead wherein a tubular guide surrounds a flexible conductor and the helix at the distal end of the conductor in the course of the implanting step. The tubular guide has a weakened portion which extends longitudinally of the conductor to the proximal end of the lead and enables the person in charge to separate the guide from the conductor and to extract the separated guide from a patient's body when the implanting step is completed. The thus exposed conductor is then rotated in order to anchor the tip of the helix in the adjacent heart tissue. Such proposal exhibits the advantage that the tubular guide and the conductor need not be provided with mating threads which renders it possible to reduce the radial dimensions of the lead. Moreover, the tubular guide reliably prevents any contact between the tip of the helix at the distal end of the conductor and the adjacent tissue during implantation of the lead, e.g., through selected blood vessels. However, the tubular guide contributes to initial cost of the lead, especially since it must be provided with a weakened portion to permit its separation from the implanted conductor.
European patent application No. 0 337 035 of David W. Mayer et al. (published Oct. 18, 1989) discloses a cardiac pacing electrode wherein the helical fixation element permanently extends beyond the distal end of the dielectric sheath. The exposed fixation element is confined in a spherical bio-compatible covering which is soluble in body fluids. The covering prevents contact between the tip of the fixation element and the tissue during implantation but is supposed to dissolve thereafter and thus permit anchoring of the fixation element in the wall of a patient's heart. Cardiac pacemaker leads which are supplied with such coverings are known as sweat tips and are distributed by Cardiac Pacemakers, Inc. of St. Paul, Minn. A drawback of these pacing electrodes is that the person in charge must permit a certain interval of time to elapse subsequent to completion of the implanting step in order to ensure complete dissolution of the bio-compatible covering prior to rotation of the conductor in order to drive the tip of the fixation element into heart tissue. The exact duration of such interval varies from patient to patient which can create problems, especially when the implanting step involves non-anticipated complications so that the implanting step takes up more time than expected. Thus, the sharp tip of the helical fixation element is likely to injure the tissue during the last stage of implantation.
European patent application No. 0 000 725 of Hans-Jurgen Bisping (published Feb. 21, 1979) discloses a cardiac pacemaker lead wherein the helical terminal at the distal end of the conductor extends beyond the distal end of the tubular sheath in the course of the implanting operation. The tissue surrounding a blood vessel and the heart tissue are protected from injury by a cylindrical body which projects beyond the tip of the helical fixation element during implantation and is thereupon retractable by a cord or wire preparatory to anchoring of the helical terminal in the heart tissue. The cylindrical body has an elastically deformable head to permit extraction from the helical terminal or is provided with external threads meshing with the terminal in the course of the implanting step. In either event, the sharp tip of the helical terminal is not concealed and the likelihood of damage to tissue by the exposed tip depends upon the accuracy of finish of the cylindrical body.
British patent application No. 2 067 411 of Carl Dorning (published Jul. 30, 1981) discloses an electrode lead wherein the distal end of the conductor is dull and the conductor is surrounded by an insulating sheath having a distal end provided with tines which are to anchor the conductor in proper position. A drawback of such proposal is that the distal end of the conductor is not reliably (positively) anchored in the tissue, e.g., in the wall of a patient's heart.